1. Field of the Invention
This invention relates to a surface treatment method of metallic material to be used for roughening the surface of metal for the purpose of preventing halation and glaring. Also, it relates to a surface treatment method of metallic material for providing an excellent adhesiveness with an organic polymer material. The present invention also relates to a metallic material obtained by such a surface treatment method. For the purpose of the invention, the term metallic material refers to any of metals, and metal parts subjected to the surface treatment.
2. Description of the Related Art
Generally, two surface treating methods are known for firmly bonding an organic polymer material to a metallic material. The first method is a method of roughening the surface of the metallic material in order to provide the surface with many small dents having a wedge effect for the organic polymer material. The second method is a method of forming a layer of a third material that is highly adhesive both to the metallic material and the organic polymer material on the surface of the metallic material.
When a mechanical process, for example a shot blasting process, is used for the first method, many small dents are formed on the surface of the metallic material by injecting hard fine particles of alumina, silicon carbide or silicon nitride onto the surface of the metallic material. With this process, the effective surface area of the metallic material for binding with the organic polymer material is increased, and the dents formed on the surface of the metallic material are filled with the organic polymer material to give rise to a wedge of the organic polymer material so that the two materials will be firmly bonded to each other.
However, such a mechanical process as shot blasting process has drawbacks in that it cannot be used when the metallic material has a profile of a thin sheet because the profile of the thin sheet can easily be changed by the shot blasting process. Another disadvantage is that the surface cannot be evenly roughened if the metallic material has a complex profile. Additionally, a metallic material subjected to a mechanical process has to be bonded with an organic polymer material immediately after the surface treatment, because the surface of the metallic material is active as a result of having its protective surface layer removed, so that the adhesiveness of the surface will gradually be lost as the surface is oxidized with time. For these reasons it is inconvenient to use this mechanical process.
When a chemical process is used for the first method, the surface of the metallic material to be treated is brought into contact with hydrochloric acid, sulfuric acid or nitric acid to chemically etch and roughen the surface of the metallic material. Since the surface of metallic material is chemically not uniform microscopically due to the difference of its metal structure and the presence of nonmetallic inclusions and crystal grain boundaries, chemically active areas will be more predominantly etched to produce many small etching pits on the surface. With this process, however, a remarkable roughening effect may not be achieved depending on the kind of metallic material. Also, small etching pits, which are produced, can be etched out and disappear when etching is performed excessively. Further, this process normally does not provide etching pits having a sharp configuration when compared with the mechanical process.
Regarding, the second method of forming a layer of a third material on the surface of the metallic material, a variety of treatment processes have been performed.
A treatment of a steel or zinc type material using phosphate solution, that of an aluminum type material using chromate solution, and that of a copper type material forming a copper oxide film can be done. Also, the application of a silane coupling agent onto the surface of a variety of different metallic material, has also been performed.
This second method of forming a layer of a third material is advantageous over the above described mechanical method because it results in auxiliary effects. As the surface of the metallic material is covered by a layer of a third material, it becomes chemically less active and less prone to rust. The surface becomes highly anti-corrosive after an organic polymer material is bonded thereto.
Particularly, in a process using phosphate layer or copper oxide layer with the second method, the metallic material will be etched simultaneously with the formation of the layer thereon, and the formed layer will comprise fine crystal grains with a size between a sub-micron and tens of several microns. Thus the surface of the metallic material is covered by many small etching pits as well as the adhesive layer, so that not only the layer itself will show an excellent adhesiveness but also the surface of the metallic material itself will show an effective mechanical bonding effect.
However, when the surface of a metallic material is etched mainly for the purpose of roughening, the structure of the layer formed on it can become coarse thereby degrading its strength and reduce its bonding force. Therefore, it is difficult in the second method to find optimal etching conditions for such a roughening operation. Accordingly, while the second method may be used for forming the layer for painting over it, it is not suitable for forming a layer having sufficient bonding force. Therefore, for the surface of a metallic material bonded with rubber or some other organic polymer material, where shearing stress and/or peeling stress may appear after the bonding operation, the first and the second method have to be used together.
The second method of applying a silane coupling agent can be used for many different types of metallic materials compared with the above described chemical process. It can be used in a simple operation of applying a silane coupling agent to the metallic material. However, the produced layer in this case will be very thin and the layer does not provide a sufficient bonding force with organic polymer materials.
As explained above, the mechanical method has a weak point in that it can be difficult to apply to the metallic material if the metallic material has a thin sheet profile. Also in this method the metallic material becomes rusty. The etching process in this method has a weak point in that a sharp etching pit is difficult to obtain. The method of forming a layer of a third material has a weak point in that the adhesive strength with an organic polymer material tends to be insufficient.
Furthermore, a roughened surface of metallic materials is required in several fields for optical reasons. Such metallic materials may have a plane surface, a curved surface or a more complicated surface. Therefore, there has been a need for a method, which will uniformly roughen the surface of a metallic material regardless of the profile of the surface.